Process and device for removal of combustion pollutants under high oxygen conditions

ABSTRACT

The subject invention is a process and device to effectively remove combustion pollutants under oxygen conditions greater than six volume percent. Microwave enhancement of oxidation/reduction catalysis is selectively employed in a multiple step process. However some of the steps do not utilize microwaves as they convert NO to NO 2  by employing a conventional oxidation catalyst, and which then in a subsequent step which does employ microwaves uses a reducing agent to undergo catalytic reduction to N 2 .

This application is a divisional of application Ser. No. 08/779,311,filed Jan. 6, 1997 and issued as U.S. Pat. No. 5,767,470 on Jun. 16,1998.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to a process and a device for removingcombustion pollutants, such as nitrogen oxides, carbon monoxide,particulate matter like soot, volatile organic compounds, and otherhazardous air pollutants. In particular the present invention isdesigned to operate under high oxygen conditions that often occur incombustion gases.

2. Background

Combustion products often contain many substances that require removalbefore release to the environment. Among these pollutants are nitrogenoxides, carbon monoxide, particulate matter like soot, volatile organiccompounds, and other hazardous air pollutants. In addition mostcombustion processes operate with considerable excess oxygen, usuallyfrom air, so that the combustion gases still contain much residualoxygen. A combustion gas containing more than six percent oxygen byvolume is deemed to produce a high oxygen condition for removal ofcombustion products.

In particular it is economically desirable to remove all combustionpollutants with one pass through an appropriate device and not have toemploy successive apparatuses to fully accomplish the needed pollutantremoval. The subject invention accomplishes this with a two stageprocess employing the selective use of appropriate catalysts along withmicrowaves created as a radiofrequency energy field.

A particular difficult combustion pollutant is nitrogen oxides presentin various forms and usually identified as NO_(x) to incorporate NO,NO₂, etc. Microwave reduction of NO_(x) proceeds well in the presence ofpyrolytic carbon, such as char and soot, provided the oxygen content ofthe gas is small, less than 6 percent. As the oxygen content of the gasexceeds this 6 percent level, the removal of NO_(x) becomes less andless efficient. Cha has shown this removal for the low oxygen situationin U.S. Pat. Nos. 5,246,554; 5,256,265; 5,269,892; and 5,362,451; andthe specifications of these patents are hereby incorporated byreference. The subject invention covers the high oxygen case.

Quantum radiofrequency (RF) physics is based upon the phenomenon ofresonant interaction with matter of electromagnetic radiation in themicrowave and RF regions since every atom or molecule can absorb, andthus radiate, electromagnetic waves of various wavelengths. Therotational and vibrational frequencies of the electrons represent themost important frequency range. The electromagnetic frequency spectrumis conveniently divided into ultrasonic, microwave, and optical regions.The microwave region runs from 300 Mhz (megahertz) to 300 GHz(gigahertz) and encompasses frequencies used for much communicationequipment. A treatise of such information is presented by Southworth,Principles and Applications of Wave guide Transmission, Nostrand, N.Y.,1950, which is herewith incorporated by reference.

Often the term microwaves or microwave energy is applied to a broadrange of radiofrequency energies, such as 915 MHz to 5000 MHz,particularly with respect to the common frequencies, 915 MHz and 2450MHz. The former is often employed in industrial heating applicationswhile the latter is the frequency of the common household microwave ovenand therefore represents a good frequency to excite water molecules.

The absorption of microwaves by the energy bands, particularly thevibrational energy levels, of the atoms or molecules results in thethermal activation of the nonplasma material and the excitation ofvalence electrons. The nonplasma nature of these interactions isimportant for a separate and distinct form of heating employs plasmaformed by arc conditions at a high temperature, often more than 3000°F., and at much reduced pressures or vacuum conditions. For instance,refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition,Supplementary Volume, pages 599-608, Plasma Technology. In microwavetechnology, as applied in the subject invention, neither condition ispresent and therefore no plasmas are formed.

Microwaves lower the effective activation energy required for desirablechemical reactions since they can act locally on a microscopic scale byexciting electrons of a group of specific atoms in contrast to normalglobal heating by raising the bulk temperature. Further this microscopicinteraction is favored by polar molecules whose electrons become locallyexcited leading to high chemical activity; however, nonpolar moleculesadjacent to such polar molecules are affected to a much lesser extent.An example is the heating of polar water molecules in a common householdmicrowave oven where the container is of nonpolar material, that is,microwave-passing, and stays relatively cool.

As used above microwaves are often referred to as a form of catalysiswhen applied to chemical reaction rates. For instance, see Kirk-Othmer,Encyclopedia of Chemical Technology, 3rd Edition, Volume 15, pages494-517, Microwave Technology.

Related United States patents using microwaves include:

    ______________________________________                                        U.S. Pat. No.   Inventor      Year                                            ______________________________________                                        4,545,879       Wan et al.    1985                                              5,087,272       Nixdorf             1992                                      5,246,554       Cha                 1993                                      5,256,265       Cha                 1993                                      5,269,892       Cha                 1993                                      5,362,451       Cha                 1994                                      5,277,770       Murphy               1994                                     5,423,180       Nobue et al.         1995                                     5,536,477       Cha et al.           1996                                   ______________________________________                                    

Referring to the above list, Nixdorf discloses using a filter containingsilicon carbide whiskers to remove particulate matter from a gas streamand then clean said filter with microwave heating. The subject inventionis not just a filter.

Cha ('554) discloses removing gas oxides by adsorption on a char bed andthen reduction by microwaves as two distinct steps. The subjectinvention uses a related process for the removal of soot and some NO_(x)in the presence of microwaves as one of several stages.

Cha ('265) discloses removing gas oxides in a homogeneous mixture withsoot carried out in a waveguide reactor. In contrast the subjectinvention does not actively employ homogeneous reduction involving sootand has a non-microwave stage.

Cha ('892) discloses pyrolytic carbon bed for removal of gas oxidesusing microwave catalysis. The subject invention does not employcollecting a pyrolytic carbon bed since soot is burned as it iscollected.

Cha ('451) discloses a waveguide reactor to efficiently performradiofrequency catalysis. The subject invention does not employ awaveguide reactor.

Murphy discloses reactivating plasma initiators using microwaves in thepresence of oxygen which is checked by a methane conversion reaction,where such plasma initiators are, or contain, metallic catalysts. Thesubject invention has no connection with the plasma regime of gases butdoes employ conventional metallic catalysts.

Nobue et al. disclose a filter regeneration system for an internalcombustion engine using microwaves. The subject invention is not just afilter.

Cha et al. ('477) disclose a pollution arrestor using a soot filterfollowed by catalytic sections, using only reduction catalysts, toremove various gaseous pollutants with the total assembly within amicrowave cavity. This pollution arrestor with only a reduction catalystdoes not perform satisfactorily under high oxygen conditions. Converselythe subject invention performs well under high oxygen conditions.

SUMMARY OF INVENTION

The objectives of the present invention include overcoming theabove-mentioned deficiencies in the prior art and performing effectiveremoval of combustion pollutants with a single device under oxygenconcentrations greater than six volume percent. The most difficultpollutant is NO and requires a special process step involving itsconversion to NO₂ before final reduction to N₂.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the device for pollution removal along with sectionals,FIGS. 1A and 1B.

FIG. 2 shows microwave soot oxidation rates in the presence of NO.

FIG. 3 shows microwave NO reduction by soot.

FIG. 4 shows microwave NO_(x) reduction under high oxygenconcentrations.

FIG. 5 shows NO oxidation with Pt/Rh zeolite catalyst.

DETAILED DESCRIPTION OF INVENTION

The pollutants associate with combustion products represent nitrogenoxides, NO_(x), carbon monoxide, CO, particulate matter less than 10microns, PM10, unburned products of combustion, volatile organiccompounds, VOC, and other hazardous air pollutants, HAP. Most of thesoot is likely present in PM10 material. In addition in the subject casethe concentration of the combustion gases contains oxygen in excess ofsix volume percent which is common in combustion processes where excessair is employed.

Traditionally the method to remove the full range of combustionpollutants involves several distinct steps. First the particulate matteris filtered out which requires replacing filters and regenerating thesubsequent clogged filters. For removal of NO_(x) selective noncatalyticreduction can be employed by using a reducing agent such as anhydrousammonia or urea injected immediately after combustion and is oftenemployed in large coal-fired power plants, but the efficiency is low.Alternatively a selective catalytic reduction is employed where thegases after addition of a reducing agent are past through a reductioncatalyst that produces nitrogen gas and water, but good efficiencyrequires high temperatures greater than 750° F. Unutilized ammonia isoften an additional problem is this conventional removal process forNO_(x).

Once particulate matter and NO_(x) are treated, then any othercombustion pollutants are oxidized in a conventional manner often by atype of industrial catalytic converter.

Because of various process conditions the conventional treatment ofcombustion gases for pollutant removal requires three operating levelsand separate equipment. The subject invention overcomes this restrictionby using a single device.

The subject invention employs microwaves which are a versatile form ofenergy that is applicable to enhance chemical reactions, since theenergy is locally applied by its vibrational absorption by polarmolecules and does not produce plasma conditions. A class of enhancedreactions are those whose reaction kinetics appear unfavorable atdesirable bulk temperature conditions, such as below about 300° F.

A unique form of microwave chemistry is microwave or radiofrequencycatalysis which is performed with char where it is formed into large bedparticles and exposed to various gases. Char is a form of pyrolyticcarbon, but is not commonly identified as soot, and is formed by drivinggases from coal in a non-oxidizing atmosphere. Common soot is oftenformed in an oxidizing gas due to incomplete combustion, but alsoappears as the degradation of hydrocarbons. Since char and soot are bothpyrolytic carbon with polar constituents, their equivalency formicrowave catalysis has been shown by Cha ('477) and such specificationis hereby incorporated by reference.

Cha ('477) performs a laboratory setup to determine the effect ofmicrowave catalysis at high wattage upon the reduction of NO to N₂ inthe presence of soot. With 1000 ppm NO in 75% N₂ and 25% air at 500watts, the 5.25% O₂ test showed good reduction of NO reaching only 16ppm after 10 minutes. Conversely with 1000 ppm NO in 50% Na and 50% airat 500 watts, the 10.5% O₂ test showed considerably poorer reduction ofNO reaching only 60 ppm in 10 minutes. Cha ('477) gives data for severalother test runs. Currently it has been determined that the appropriatebreakpoint is about 6% oxygen; thus, more than 6% volume percent oxygenis designated as high oxygen conditions and is the subject of thesubject invention.

FIG. 2 is a plot of microwave soot oxidation rates in the presence of1000 ppm NO and oxygen concentrations above 6 percent at wattage valuesbelow 150 watts using the laboratory setup of Cha ('477). The 14% O₂data well indicate good oxidation rates. FIG. 3 gives the result fromthis same set of tests of the poor NO reduction by microwave catalysisand soot under these high oxygen conditions. Only the 50 watt test datashow even reasonable results. For both FIG. 2 and FIG. 3 the testconditions were 1000 ppm NO in inlet gas with a total gas flow rate of 5SCFH, and initial soot of 500 mg in an 0.5" ID quartz tube using awaveguide reactor.

The conclusion is that microwave reduction of NO by soot under highoxygen conditions is poor. Thus additional catalytic processes arerequired to reduce NO concentrations to acceptable levels in combustiongases under these high oxygen conditions.

FIG. 4 shows the test results of microwave destruction percentages usingplatinum/rhodium (Pt/Rh) catalyst. Little dependence was found over the10, 12 and 14 percent oxygen concentrations in the inlet gas. The testconditions were an inlet gas containing 1000 ppm of NO_(x), which wasabout 90 ppm NO and 10 ppm NO₂, and 1000 ppm JP-8 was passed through a5.5-inch long 1-inch diameter of Pt/Rh zeolite to oxidize approximatelyone-half of the NO to NO₂, and then a second 1-inch tube containing abed of Pt/Rh zeolite mixed with SiC was employed to reduce the NO₂ to N₂using a waveguide reactor. The Pt/Rh catalyst was an oxidation/reductiontype made by United Catalyst and designated PRO-VOC 7 Zeolite catalystor its equivalent; further, it is best to employ a zeolite substrate forsuch catalyst, as the commonly used alumina or clay substrate is lesseffective in similar tests. The zeolite, silicon carbide mixture isrequired in order to absorb microwaves since zeolite itself isnon-microwave absorbing. The JP-8 is a reducing agent. The resultsindicate that reasonable NO₂ destruction occurs, about 50 percent at lowmicrowave power with decreasing amounts at higher powers. This suggeststhat at least two such stages, and likely more, are required to approacha 90 percent reduction of NO which is the ideal target requirements.

FIG. 5 shows the oxidation of NO to N₂ using Pt/Rh zeolite catalystunder various high oxygen concentrations or conditions without usingmicrowaves; thus, room temperature was present. The test conditions were51.00 grams of Pt/Rh zeolite catalyst presaturated with NO and NO₂ in aone inch quartz tube with 1000 ppm NO in 10 SCFH purge gas. Adequateconversion of NO to NO₂ resulted but it was evident that in manysituations more than one stage would be required.

The subject invention thus requires both microwave and non-microwaveprocesses. For a microwave absorbing region the catalyst formed on azeolite substrate is mixed with SiC; for a non-microwave absorbingregion, no SiC is employed.

The subject invention as a single device must remove soot as a firststep before it starts additional removal of other combustion pollutantssince soot can foul any catalysts employed if microwave energy is notpresent. The PM10, which is largely soot, is removed with a ceramicfilter to stand high temperature conditions and is burned withmicrowaves in place on said filter since high oxygen conditions arepresent. The reaction is:

    C+O.sub.2 ---(RF)--->CO.sub.2                              (1)

where ---(RF)---> implies that RF microwave energy catalyzes thereaction to proceed in the direction indicated at a lower bulktemperature than normal burning would transpire. A convenient bulktemperature is about 300° F., but temperatures up to about 500° F. areoften utilized. Periodic microwave energy is sometimes utilized underhigh soot conditions since this reaction is highly exothermic so timefor cooling is needed. A small amount of microwave catalytic reductionof NO_(x) to N₂ does occur under these high oxygen conditions incontrast to the low oxygen condition, under six volume percent, wheresuch microwave catalysis readily occurs.

The second section of the first stage of the process reacts NO to theNO₂ chemical form which is required in the subsequent section in orderto be reduced by a conventional oxidation/reduction catalyst under thesehigh oxygen conditions. This section employs a conventional oxidationzeolite catalyst to insure substantial NO is converted to NO₂ and thisstep is performed with minimal microwave interaction since the catalystsubstrate employed is non-microwave absorbing.

The third section of the first stage is a microwave enhanced reactionusing a conventional oxidation/reduction catalyst, such as Pt/Rh, with areducing agent. This reducing agent is a thermally stable hydrogencontaining material, such as No. 2 distillate fuel oil or its commonequivalent, JP-8. A significant amount of the NO₂ formed in the previousstep is reduced to N₂. Other oxidizable pollutants are partiallyoxidized in this high oxygen atmosphere.

In order to reduce the NO_(x) concentration to low enough levels to bereleased to the atmosphere which generally requires a 90 percentreduction, a second stage is added to the subject device. This stage twois an additional two step catalysis performed under these high oxygenconditions repeating the second and third steps of stage one, and uses aconventional catalyst of the oxidation/reduction type, such as Pt/Rh.The first section of stage two oxidizes a significant amount of theremaining NO to NO₂ under non-microwave conditions which is then reducedto N₂ in the next section as further oxidation occurs of the remainingpollutants. Any excess JP-8 is treated as another pollutant to befurther oxidized. In order for the Pt/Rh catalyst to perform the neededoxidation/reduction, it requires energy from microwaves, so the catalystparticles are immersed within a bed of silicon carbide that is a goodmicrowave absorber. Again the Pt/Rh catalyst is made with a zeolitesubstrate. For the non-microwave conditions no SiC is employed.

The properties of No. 2 distillate fuel oil are reported in Perry,Chemical Engineers' Handbook, 6th Ed., pages 9-10 to 9-13, and thisinformation is hereby incorporated by reference. An important propertyof JP-8 or any reducing agent is that it be thermally stable at theabout 500° F. temperatures involved. Further its volatility is such sothat it can be injected as a liquid but quickly evaporate into a vaporas it mixes with the gases. The important properties of JP-8, adesignation established by the United States Air Force, include aspecific gravity, 60/60° F., in the range of 0.788 to 0.845; a hydrogencontent of a minimum of 13.5 weight percent; a boiling range of 300 to626° F.; a freezing point of a maximum of -58° F.; and an aromaticcontent of a maximum of 25 volume percent. In general a reducing agentis employed selected from the group consisting of normal-hexane,iso-hexane, JP-8, number two grade of fuel oil, and combinationsthereof.

After leaving the second stage the exit gas has a substantial portion ofnitrogen in the N₂ state and all combustible gases are substantiallyburned, so that the exit gas is substantially free of pollutant CO andcontains ideally no more than ten percent of the original NO_(x).

FIGS. 1, 1A, and 1B show the subject invention as a single device whichtreats polluted combustion gases.

FIG. 1 shows the complete device with its two stages.

FIG. 1A is a cross section at A--A, while FIG. 1B is a cross section atB--B. The device consists of stage one 10 and stage two 20. The pollutedcombustion gases 9 enter and are injected 11 with a small amount ofreducing agent JP-8 to mix well with the gases. The gases then enter aceramic filter 12, such as Nextel type made by 3M Company or equivalent,which removes PM10 which contains much soot that easily absorbsmicrowaves. After soot removal by filtering and burning the gases passthrough a Pt/Rh catalyst bed 13 which has a non-microwave absorbingsubstrate containing no SiC and this bed oxidizes NO to NO₂. The JP-8reducing agent passes through unreacted since this bed does not absorbmicrowaves. This is followed by another bed of Pt/Rh catalyst 14 on amicrowave absorbing zeolite substrate containing SiC, and thisselectively oxidizes and reduces No_(x), CO, VOC, and HAP since muchoxygen is present.

Stage two 20 has additional JP-8 injection 21 as the gases enter a Pt/Rhcatalyst bed 23 again with a non-microwave absorbing substrate wheremuch of the remaining NO is oxidized to NO₂. Because microwaves are notabsorbed in this section, the reducing agent JP-8 passes throughunreacted. The last section is again a Pt/Rh catalyst bed 24 on amicrowave absorbing zeolite substrate where final reduction of NO₂occurs as well as oxidation of any remaining JP-8.

Surrounding the total device within the structurally supporting shell 30is a general microwave input point 31 that effectively distributes themicrowaves throughout the device using an antenna or helix waveguide 32.

FIG. 1 shows a 24 inch height to stage one and a 16 inch height to stagetwo; however, these were just convenient measurements during thebuilding of the initial prototype. An actual production model would besized in accordance with variables of the combustion gases stream, suchas the flow rate and concentration of pollutants of said gases, in orderto obtain a reasonable pressure drop and to reduce the level ofpollutants to an acceptable value. Further the geometric shape of thedevice is not critical as the important aspect is that the varioussections as well as the stages be positioned in a serial arrangement.Thus one large cylindrically concentric system is employable but otherequivalent arrangements are also useable.

As a general description the device for removal of pollutants from acombustion stream comprises an encompassing structural shell containinga microwave energy field thus forming a microwave cavity. Within saidshell is positioned several serial stages the first of which collectsand burns soot on a ceramic filter. This filter may contain siliconcarbide to enhance the soot burning. Following said filter is aplurality of additional stages each consisting of an oxidation/reductioncatalyst, such as Pt/Rh or equivalent, on two distinct substratespositioned serially: (a) a microwave-passing substrate, and (b) amicrowave-absorbing substrate. The microwave-passing substrate containsno carbon and is zeolite based or equivalent. Zeolite is a class ofminerals that are hydrated aluminosillicates. The microwave-absorbingsubstrate contains carbon, preferably carbon containing polar molecules,and is silicon carbide or equivalent, but also may contain zeolite. Themicrowave-absorbing substrate has the additional provision of allowing,but not requiring, reducing agent injection. An equivalent bed can bemade using SiC foam with fine catalyst material interspersed within itsupported on an alumina or zeolite substrate.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingcurrent knowledge, readily modify and/or adapt for various applicationssuch specific embodiments without departing from the generic concept,and therefore such adaptations or modifications are intended to becomprehended within the meaning and range of equivalents of thedisclosed embodiments. It is to be understood that the phraseology orterminology herein is for the purpose of description and not oflimitation.

I claim:
 1. A device for removal of pollutants from a combustion streamwhich contains more than six percent oxygen by volume comprising:anencompassing structural shell; means for containing one or moremicrowave cavities within said shell to effectively distributemicrowaves; a first stage soot collecting and burning silicon carbidefilter within a first microwave cavity; a second stageoxidation/reduction catalyst on a zeolite substrate; and a third stageoxidation/reduction catalyst on a microwave-absorbing substrate within asecond microwave cavity with provision for reducing agent injection. 2.The device according to claim 1 wherein said microwave-absorbingsubstrate comprises a catalyst substrate containing carbon molecules. 3.The device according to claim 2 wherein said catalyst substratecontaining carbon molecules comprises silicon carbide.
 4. The deviceaccording to claim 1 wherein all microwave cavities compriseradiofrequency energy selected from the frequency range consisting of915 to 5000MHz.
 5. The device according to claim 1 wherein all saidoxidation/reduction catalysts comprise platinum/rhodium.
 6. The deviceaccording to claim 1 where said first microwave cavity and said secondmicrowave cavity comprise a unified microwave cavity.